Oil seals have been in use for many years to prevent leakage from oil reservoirs and are interposed between a shaft and a housing, one of which rotates. The problem has been that even with an oil seal that theoretically should be perfect, there has been leakage due to imperfections of the shaft surface. For example, a scratch along the shaft can let oil leak out, under both static and rotating conditions. In some instances the scratches or imperfections are such as to increase leakage during rotating conditions.
To combat this, shaft seals with hydrodynamic action have been used. Some such seals exhibit the desired function in only one direction; for that one direction of shaft rotation they utilize such things as spiral grooves or flutes formed on the air side of an elastomeric sealing lip to return oil from the air side of the oil seal to and beyond the lip, back into the oil reservoir. Such oil seals are hydrodynamic in only one direction of rotation of the shaft, in the opposite direction they are either ineffectual or actually increase seal leakage.
However, in many installations it is difficult to predict which will be the direction of rotation relative to the seal,--for example, a rotating shaft with the same seal at each end, mounted symmetrically. Furthermore, some shafts rotate at times in one direction and at other times rotate in the opposite direction. For both these reasons oil seal manufacturers have devised various bidirectional hydrodynamic oil seals. Here again, the air side of the sealing element is provided with flutes or other effective configuration to act when the shaft or bore rotates in a manner such that rotation in either direction has the effect of returning to the oil reservoir oil that may have leaked past the lip.
Thus, in molded elastomeric seal elements, hydrodynamic action, whether unidirectional and bidirectional can be obtained by molding certain particular shapes on the air side of the lip. However, when seal materials such as polytetrafluoroethylene are used, molding is not easy and in the past has been costly. Hence, the normal polytetrafluoroethylene lip is a simple flat wafer type of washer or annulus, and to provide it with hydrodynamic action has involved additional steps in manufacture, e.g., machining or cold forming a configuration on the wafer. There are other occasions and materials when the use of a simple wafer-type seal is desirable.
An object of the present invention is to provide simple wafer-type oil seals with bidirectional hydrodynamic action. Another object is to do so without providing any special formation on the sealing lip.
Another object of the invention is to provide a bidirectional hydrodynamic polytetrafluoroethylene seal without any flutes.
Another object of the invention is to provide hydrodynamic action which is directly proportional to the rotational speed of the rotating element.
Another object of the invention is to provide a hydrodynamic seal operating in both directions, and not requiring any garter spring or similar element to maintain contact between the lip and the shaft or bore with which the lip is moving relatively.
Another object of the invention is to provide a seal with a rotating wafer lip that employs centrifugal force to help achieve it hydrodynamic action.
Some structures look somewhat like the structure of the present invention, but do not operate in the same manner. For example, German patent DE No. 3 031 870 is a grease seal of a type called in that patent a "cassette seal". Although it bears some superficial resemblance to the seal of the present invention, its action is quite different because the wafer lip is intended to exclude dirt rather than retain oil, because the lip is mounted on a stationary shaft, and is wiped by a rotating bore wall, and because the sealing edge faces the air side of the seal. The seal lip is not rotated and does not generate the centrifugal force which the present invention uses as an important element in both preventing seal leakage and in providing hydrodynamic return of any oil that might have been leaked. Moreover, a grease seal like that of this German patent is different in purpose from the oil seal of the present invention, for it is often desirable to flush grease through a grease seal when adding lubricant, whereas it is not desirable to flush oil through an oil seal.
Another grease-purgeable seal is shown in U.S. Pat. No. 4,106,781. There again, the seal is intended to exclude dirt and dust and other foreign matter and is also intended to permit the passage of the lubricant through the "seal". Again, the seal faces the air side rather than the grease side. It is not clear from that patent which element rotates, but it makes no difference, because centrifugal force is not employed to augment sealing action. Moreover, it is believed that it is intended to be a stationary cassette-type seal like that of the German patent.